Internal combustion engines are designed to operate over a range of rotational speeds. The lowest rotational speed, usually referred to as idle speed, is largely decided by the minimum sustainable rotational speed for combustion, given limitations on flywheel size, etc. The maximum rotational speed is partly constrained by the increasing inertia forces arising from rapidly rotating components and partly from the limits of combustion speed. All engines need to be controlled in some way to ensure that their rotational speed remains in this “envelope”. Too low a rotational speed at any time during operation will risk engine stalling, which may have operational consequences ranging from inconvenient to disastrous. Whereas too high a rotational speed may cause structural failure of components and rapid destruction of the engine. For all engines, whether coupled to a load or not, an increase in engine rotational speed leads to an increase in energy losses from friction and viscous drag, etc.
For a throttled, carbureted petrol engine the extra air/fuel mixture drawn into the engine as a result of an increase in engine rotational speed is substantially less than the increase in rotational speed, resulting in a stable situation. At a fixed throttle setting such an engine will settle at a substantially constant rotational speed. With an applied load, such as when driving a vehicle direct to the road wheels, or a propeller on an aircraft, the system remains stable: the increase in engine load means that the throttle has to be set to a different position (more flow area for an increase in load or required speed) but this is the only input required from an operator. Those gasoline engines that use fuel injection have different requirements but typically have some form of governing built in, either electronically on modern systems, or generally using flyweights for older mechanical systems.
Unlike gasoline engines, diesel engines typically require some form of governor. This is because the different way the fuel is supplied leads to a different reaction to changes in engine rotational speed. For most diesel engines, the airflow through the engine is sufficient at all times to burn more fuel than is available from the fuel injection pump. The fuel delivery from the fuel injection pump therefore controls the engine's output and rotational speed.
For most types of fuel injection pump, the pump's volumetric efficiency, hence fuel quantity per injection at fixed setting, is designed to be greatest at mid-engine rotational speed to satisfy other operating requirements. Generally the engine's combustion efficiency will rise with increasing engine rotational speed, again the peak combustion efficiency usually occurring at mid-engine rotational speed.
Thus a small increase in engine rotational speed at idle results in more torque being available from the engine because of the greater fuel quantity being injected per injection; typically this increase in torque is greater than the increase in torque losses from viscous drag, etc. This is an unstable situation and such an engine will increase in rotational speed until the combination of fuel delivery and combustion characteristics changes so that torque produced is equal to torque absorbed.
When such an engine is connected to a load, the characteristics of the load influence the situation. Some loads have a more or less constant torque requirement as rotational speed increases, others, such as propeller loads, have rising torque characteristics.
Those loads that are more or less constant in torque, e.g. a slow heavy vehicle along a flat road, still result in an unstable situation. Such a vehicle would only be controllable because the driver notes an increase in vehicle speed and adjusts his input to the fuel pump. For a heavy vehicle the response time is long and a driver would easily be able to respond in time.
Those loads that increase in torque as engine rotational speed increases, e.g. a fast vehicle along a flat road, or driving a propeller, may well result in a stable system and have no requirement in such a situation for a governor. However a change in circumstances may well result in instability returning and a governor being required.
The usual requirement of a governor for accurate rotational speed control at idle without “hunting”, requires mechanical governors to use substantial flyweights to provide enough change in force with small changes in engine rotational speed to be sure to overcome the friction forces present in the mechanism. This results in a heavy, bulky and costly mechanism with oil lubricated, accurately made parts to ensure low friction, an enclosure to keep the mechanism clean, and adjustment screws to enable idle speed to be accurately set.
Highly rated diesel engines require some form of supercharging to increase airflow through the engine hence enable more fuel to be burnt. Currently the preferred system is to use turbosupercharging, which gives the best compromise in fuel consumption, cost, complexity reliability and durability.
Four-stroke diesel engines do not require a positive pressure in the intake manifold (airchest), with respect to atmospheric pressure, under all conditions. Typically there will always be a slight pressure in the intake manifold relative to the pressure at the entry to any supercharger that may be fitted to the engine.
All two-stroke diesel engines do require a positive pressure across the cylinders at all times. It is this pressure that drives the airflow through the engine, providing fresh air to the cylinders to be burnt at each cycle. This requirement means that there is a positive pressure in the intake manifold (airchest), with respect to atmospheric pressure, at all times when the engine is running.
Given that the inlet ducting as far as the first stage of a supercharger can only be restrictive, the pressure difference between the intake manifold (airchest) and the point immediately before the first supercharger in the system will be slightly higher than the pressure difference between the intake manifold and the atmosphere. It is in the nature of all simple 2-stroke engines that these pressure differences will always increase as engine rotational speed increases.
For aircraft and other applications where weight and space are at a premium the use of a traditional governor with its heavy flyweights can be a severe disadvantage, yet electronics and electronic governors bring their own disadvantages such as cost, complexity and reliability.
Some fuel pumps use a hydraulic governor instead of flyweights; these pumps usually use the principle that fluid flow resistance increases as flow velocity increases. This results in a falling volumetric efficiency of the pump with engine rotational speed and simplifies control. However these pumps have their own inherent disadvantages that may make then unsuitable for certain applications.
In the past some diesel engines have been produced with throttles as part of a simple fuel control system. The throttle was controlled directly by the operator. The inlet manifold (airchest) depression was then used to move an actuator that was linked directly to the fuel control on the injection pump. The actuator was connected such that reducing inlet manifold pressure (i.e. increasing depression) moved the fuel control to less fuel. This resulted in a relatively simple, robust system that was stable under all usual operating conditions without the cost and complexity of a flyweight type of governor. However, this system is unsuitable for use with a highly rated, supercharged engine or with a 2-stroke and brings inherent disadvantages in terms of fuel consumption, oil consumption, transient response and emissions.